Cathode heater for electron discharge devices



July 13, 1965 w. A. HAssx-:TT 3,195,004

CTHODE HEATER FOR ELECTRON DISCHARGE DEVICES Filed Aug. 19. 1960 fi/Ma mammalian :mag

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@ william A. Hassett United States Patent O 3,195,004 CATHDE HEATER FR ELECTRN DESCHARGLE DEVICES William A. Hassett, Reading, Pa., assigner to Radio Corporation 'of America, a corporation of Delaware Filed Aug. 19, 1966, Ser. No. 50,796 1 Claim. (Cl. 313-349) This invention relates to electron discharge tubes and more particularly to insulated heaters for indirectly heated cathodes of such tubes.

In many diverse types of electron tubes which use indirectly heated cathodes, 'it is the practice to employ a hollow sleeve for the cathode which is heated by a heating element contained Within the sleeve and supported therein by insulated contact with the cathode sleeve walls. The heating element is made up of a relatively ne wire of refractory material, usually of tungsten, which is folded or coiled to pack a required amount of the wire into the space enclosed by the cathode. The amount and size of the wire is determined by the power required to heat the cathode and by the desired voltage-current relationship of the heater to give this amount of power. Ends of the heater wire extend outwardly from the cathode sleeve to electrical connecting points whereby electrical current may be passed through the heater wire. The heat produced in the heating element is transferred to the cathode, mostly by radiation, and the cathode is heated thereby to its operating temperature. Since the heater and the cathode are generally operated at different electrical potentials, however, the heater wire must be coated with a suitable insulating material to electrically insulate the two elements.

A problem which has long existed in the manufacture of electron tubes using indirectly heated cathodes is to provide satistactory means to electrically insulate .the cathode from its heater. The methods most satisfactory in terms of the electrical performance of the tubes, and which are used in especially ditlicult tube types are to suspend the heater Wires within the cathode in such a manner that the wires do not touch the cathode sleeve, or to insert the heater into a separate insulating block to space the wires from the cathode. These constructions suffer from the disadvantages of requiring the use of extra tube parts, and of requiring excessive time to heat the cathode to operating temperature from the moment the tube is turned on.

To avoid these disadvantages, a construction frequently employed is to coat the heater wires with a thin coating of insulating material and to allow the insulated heater to contact the cathode walls. 'In contrast to the suspended heater constructions, the insulated heater arrangement permits rapid initial heating of the cathode. The absence of the extra insulating block reduces the mass to be heated and hencethe time required for the heating, while the contact of the thin coating kwith the cathode permits an additional transfer of heat by conduction to the cathode during the time theheater is heating up to the temperature at which heat transfer by radiation becomes more effective.

l The disadvantage of the coated heater construction, however, is the dihculty of providing a heater coating material which is satisfactory for use in an electron tube. In modern-day electron tubes, for example, the cathodes may operate at a temperature of around 800 C., with the heaters necessarily at an even higher ternperature. Furthermore, during the processing of such tubes, the temperature of the heater may reach values in the order of 2000 C.

The environment created by these elevated temperatures impose exceptionally stringent demands upon the characteristics required of materials to be used as heater wire insulation coatings. Some of the more important of the requirements are listed and brietiy described as follows: the material must be chemically and physically stable .so that it will not break down during many thousands of hours of operation of the electron tube; the vapor pressure must be low so as not to adversely affect or poison the electron emission from the chemically sensitive cathode; the material should be chemically inert so as not to interact with either the cathode sleeve or the heater wire to change the operating characteristics of either; the diffusion rate of ions and impurities through the coating by `electrolysis must be small to prevent flow of current thereby, the material should have good adherence to the heater wire to avoid peeling or iiaking dueto externally caused vibration of the heater and unequal thermal expansion of the heater wire and the coating; the electrical resistance between heater and cathode must be over l0() megohms with coating thicknesses of the order of 0.00G3-.006 of an inch; and, of course, the expense of using the material should be low.

Through the years, many materials have been tried and developed for use as heater insulating coatings, but most have been discarded as being delicient with respect to one or more of the above requirements. The material which has proven the most satisfactory of all, and which is used in practically all tubes using coated heaters, is aluminum oxide. This material, as will be described hereinafter, has the disadvantage of having a low heat radiating capacity.

The heat radiating capacity of an incandescent body is a function of the thermal radiant emissivity of the body. Furthermore, the emissivity of such a body is related tothe color, or blackness, along with the roughness of the surface of the body. It follows, therefore, that the thermal radiant emissivity of heaters used in electron tubes is determined by the insulating material coated onto the heater base wire. The significance of this is that in orderto raidiate a required amount of heat to raise the temperature of a given cathode to its electron emissive operating temperature, a heaterwith a low thermal emissivity must yoperate at a higher temperature than a heater with a higher emissivity.

Moreover, to obtain long yheater life and reliable performance thereof, it is desir-able to operate electron tube heaters rat the lowest possible temperature consistent with the heating requirements of the cathode. High `heater operating temperatures may cause an acceleration of the embrittlement of the heater wire which roccurs during the life of an electron tube. Such yembrittlement often results 4in failure of the electron tube due to breakage of the heater wire caused by mechanical shocks to which the electron ytube containing the heater may be subjected during its operation. Also, because of the different thermal expansions of the heater wire and the coating adhering thereto, large thermal growths caused by high temperatures can cause excessive `tresses between the wire and its coating which may also lead to wire breakage.. Likewise, excessive growth of the heater may result in electrical shorting between uncoated portions of .the heater wire and the cathode sleeve due to twisting or warping of the heater wire structure.

To avoid the above problems, the temperature at which the heaters must operate should be reduced by using heater insulating materials having high thermal radiant emissivities. However, because of the other requirements of heater insulating material, as mentioned, the insulation naterial almost invariably used in modern-day electron tubes is aluminum oxide. This material, unfortunately, is white in color and has a verv low thermal emissivity at the temperatures at which the heaters operate. Also, it has long been believed that the aluminum oxide used must be as pure as possible to obtain the best insulating characteristics. The purer the aluminum oxide, the whiter is it color.

it is an object of this invention to provide a cathode heater of improved designs which can be operated at lower temperatures for a given amount of heat radiation.

More specifically, it is an object of this invention to provide a heater having a coating with improved insulating characteristics and having a high thermal radiant emissivity.

A still further object of this invention is to provide an improved heater having a longer life than presently available heaters and in which shorts occurring between heater and cathode sleeve are reduced.

ln accordance with this invention, tungsten is added to the coating of aluminum oxide normally applied to a cathode heater. The result is an insulated heater with a coating thereon having insulating characteristics almost that of aluminum oxide, but with a higher thermal radiant emissivity than aluminum oxide.

The tungsten may be added to the aluminum oxide in the following ways: it may be admixed with the aluminum oxide and applied as a single coating directly onto the heater wire; it may be added as a second coating over a iirst coating of aluminum oxide applied directly to the heater wire; or it may be applied in chemical or physical combination with some other material and subsequently processed to remove the other material leaving only the desired additive material as a second coating over a rst coating of aluminum oxide. Furthermore, the second coating may comprise only the tungsten, or the tungsten admixed with aluminum oxide.

rthe invention will be more clearly understood, and further features thereof will become apparent as this disclosure proceeds with reference to the accompanying drawing wherein:

FIG. l shows an indirectly heated cathode in perspective partly broken away to show details of construction and a coated insulated heater therein of the type which may be made in accordance with my invention;

FlG. 2 is a graph showing a relation between the temperatures of black and white heaters and their cathodes with respect to the heater watts input;

FIG. 3 is an enlarged view of a portion of the heater shown in FIG. l;

FlG. 4 is a view similar to FIG. 3 but shows an alternate construction;

FIG. 5 is an idealized view of a greatly enlarged crosssection of a heater coating made in accordance with this invention; and

FIG. 6 is a view similar to PEG. 5 but shows a different heater coating made according to this invention.

Many diverse types of cathodes and heaters are employed in electron tubes, and FIG. 1 shows an example of but one of such cathodes and heaters used and well known in the art. The cathode element comprises a tubular hollow sleeve lt? having coated thereon an electron emissive material lll, which is capable of emitting electrons when it is heated to a sufficiently high tem- Cil perture. This coating may comprise 'a known mixture of strontium and barium carbonates, or the like. The heater ltshown for heating the cathode is one which is known asv a folded heater, comprises a plurality of folded strands of refractory wire, usually of tungsten, which are coated with an insulating material l2 to prevent electrical shorting between the base wire i3 and the cathode sleeve itl. Ends of the heater wire l5 extend outwardly from the cathode and are welded to electrical connectors itl, which in turn are connected to an electrical energy source7 not shown. The heater wire is coated over its entire length except at the leg portions lo and at the folded apices i8. The uncoated leg portions lo are provided to permit welding of the base wire to the electrical connectors 2.o, and the uncoated apices i8 occur as a result of the manufacturing processes of a heater wire wherein the wire is irst coated and then folded over knife edges. The coating material is somewhat brittle, and in the folding operation the coating at the point where the heater wire is bent chips off to leave the uncoated apices.

Furthermore, to prevent contact between the apices `of adjacent strands to avoid electrical shorting of the strands, it is customary to vary the length of the wire strands to stagger the apices where they might otherwise tend to contact, as shown at the upper end in FIG. 1. There may be many more strands of the wire than shown. A problem created by such staggering, however, is that when the heater is energized and heated to its operating temperature, the thermal growth of the strands of different length are unequal with the result that the heater wire structure may be twisted and distorted out of shape. If the difference between strand growths is suiiiciently great, the distortion of the heater may be such that the uncoated apices are bent over and brought into contact with the cathode sleeve. in some electron tubes, this condition is so prevalent that it is necessary to coat the apices after the heaters are inserted into the cathode in a separate coating operation, which adds additional expense to the electron tube. Reduction of the heater operating temperature in accordance with this invention, however, greatly reduces the incidence of this problem since the thermal growth and hence the difference in thermal growths of the heater strands is minimized.

Other types of heaters, such as coiled heaters, and the like, do not have apices, and accordingly, do not have this problem. Such other heaters, nevertheless, are still susceptible to breakage of the heater wire due to the embrittlement of the heater wire and the stresses formed between the insulating material and the base wire caused by excessive heater temperature as described.

Regardless of the type of heater used, therefore, reduction in heater operating temperatures is always desirable and serves to improve the reliability of heater performance.

As shown in FG. l, the heater is contained almost entirely within the cathode sleeve lil. Thus, except for the small amount of heat radiated from the heater out of the open ends of the cathode and that which is conducted from the heater through the electrical connectors Ztl, all the heat produced in the heater wires is transferred to the cathode. The cathode, in turn, dissipates all the heat energy it receives by radiation to its environment, through conduction to the elements supporting and fixing the cathode (not shown), and as thermal energy carried off by the electrons emitted from the cathode emissive coating.

In accordance with basic scientific principles, the rate at which a body radiates heat is a function of its temperature and its size. Thus, with a given cathode of an electron tube, the rate at which heat must be added to the cathode at thermal equilibrium to maintain its temperature at that level required for ecient electron emission is equal to the amount of heat which the cathode will dissipate at the desired temperature. This, in turn, determines the amount of electrical energy which must be put into the heater, and the temperature at which the heater must operate.` What occurs qualitatively is that as the electrical energy is converted to heat within the heater wires, the temperature of the heater continues to rise until the heater reaches a temperature at which its rate of energy radiation output equals the rate of energy input. The eiiiciency of such heat radiation is a function of the thermal radiant emissivity of the heater, and the higher the emissivity the lower the need by the temperature of the heater to radiate the given amount of heat.

FIG. 2 is a graphic illustration of the temperature relationship between a black heater and its cathode, and a white heater and its cathode shown as a function of electrical input energy in watts to the heaters. The cathodes are identical; and for any given energy input into the heaters, as shown, the cathodes reach substantially the same temperature while the black heater operates at a signilicantly lower temperature than the white heater.

The slight difference between the cathode temperatures is due to the differences in heat loss due to conduction. Such conductive losses are a function of temperature gradients, and the heat conducted away and lost from the black heater by its electrical connections is less than that from the hotter white heater.

For the purpose of producing heaters which may operate at the lower temperature of the black heaters as in FIG. 2, tungsten is added to white aluminum oxide coated eaters to raise the thermal radiant emissivity of the heater surface. It is noted that a body. need not necessarily be optically black in appearance, but may be thermally black even if lightin color. The reasons for this are not completely understood. Also, since the total radiation from a body is dependent upon its surface area, a body having a surface of iinely divided particles will have a larger radiating area and hence a greater thermal radiant emissivity than a body with a smoother surface.

Properties of tungsten which permit its use are that it is relatively thermally black, and that it may be added to the aluminum oxide in finely divided particle form. Also, tungsten is refractory (that is, has a melting temperature above 2000" C. and does not melt during tube processing), is chemically and physically stable, has a low vapor pressure, and does not affect the aluminum oxide so as to interfere with the insulating characteristics or the aluminum oxide.

The tungsten may be added as a second coating on top of a tirst coating of aluminum oxide (FlG. 3) as follows: heaters coated with aluminum oxide are manufactured by conventional means with the sole exception that the heaters need not be tired in hydrogen to sinter the aluminum oxide to the base wire. Actually, the heaters may be tired if desired, but this is not necessary as will be seen. The tungsten is prepared as a powder of iinely divided particles as by ball milling or the like, and mixed with a suitable dispersing agent such as methanol. This suspension is sprayed onto the aluminum oxide coated heaters by'conventional spraying means and then allowed to dry. After drying, the doubly coated heaters are tired in a hydrogen atmosphere. The time and temperature of the tiring is not critical, and for example, the ring may be for ive minutes at l700 C.

The hydrogen firing drives off the methanol dispersing agent and sinters the two coatings to provide good adherence of the aluminum oxide to the base wire, and good adherence between the tungsten second coating and the aluminum oxide first coating.

En some instances wherein it is desirable that the adherence between the two coatings be especially strong, it is best to mix a quantity of aluminum oxide powder with the tungsten mixture spray. The proportions of the mixture are not critical and depend only upon the compromise desired between the reduced radiant emissivity of the second coating and the increased adherent strength between the two coatings as caused by the addition of the aluminum oxide. The result of such a mixture (FIG. 4) is a heater having a rst coating of aluminum oxide with a second coating of a mixture of aluminum oxide and tungsten. In one instance, the proportions were 50% tungsten and 50% aluminum oxide, by weight.

Another method of application of the ytungsten coating is to add the tungsten as a compound in solution, and to apply subsequent treatment to reduce the compound and remove all but the desired tungsten from the heater.

An example of this latter method is to add tungsten to the heater as a solution of ammonium tungstate in water by spraying, and then to tire the heater in hydrogen at 1000" C. for twenty minutes to reduce the ammonium `tungstate to pure' tungsten and ammonia. The tungsten remains on the heater as a finely divided powder coating, while the ammonia is driven off as a gas.

A modication of the above which has been found to produce excellent adherence between the -two coatings is to add aluminum oxide to the ammonium tungstate solution.

One means used to accomplish this was to mix 100 grams of Nortons 38-900 aluminum oxide with 50 cc. of -ammonium tungstate solution. The ammonium tungstate solution was prepared by adding 63 grams of tungstic oxide to 113 cc. ammonium hydroxide and 200 cc. distilled water. This mixture was all-owed to stand for sixteen hours and decanted. The reaction that takes place is:

The mixture of aluminum oxide and ammonium tungstate solution was allowed to dry, and then fired in a hydrogen :atmosphere at l000 C. for twenty minutes to permit the ammonium tungstate to reduce to tungsten according to the following reaction:

The mixture ot aluminum oxide and tungsten was then ball milled for thirty minutes as the following suspension: 108 grams aluminum -oxide and tungsten; 200 cc. binder; and 100 cc. methanol. The binder was composed of 64.5 grams nitrocellulose, 1455 cc. butyl acetate; and 1355 cc. diacetone alcohol.

The above mixture was then sprayed by conventional means on the surface of aluminum oxide coated heaters. The heaters were red in hydrogen at 1000 C. for one hour to sinter the double layers together and to burn out the binder in the suspension.

After spraying and firing, the resulting coating is believed to look as represented in FIG. 5. The aluminum oxide particles prepared mechanically are of much larger size than the tungsten particles which are precipitated from their solution as a tine powder. It is believed probable that the tungsten is mixed with the aluminum oxide particles as shown, and that the basic matrix of the second coating is made up of aluminum oxide interspersed with the tungsten particles. The second coating thus physically resembles the first coating of aluminum oxide and excellent adherence between the two is achieved. The tungtsen particles serve to blacken and roughen the second coating, and a high radiant emissivity is also obtained.

For applying the tungsten and the aluminum oxide as a single coating to the heater wire, the tungsten is ball milled along with aluminum oxide to produce a mixture as represented in FIG. 6. The larger the tungsten part-icles and the greater the percentage of tungsten in the mixture in comparison to the aluminum oxide particles, the greater is the radiant emissivity of the coating. The insulation properties of the aluminum oxide will be somefwhat adversely alected by the inclusion of the tungsten, however, and darkened single surface insulation coatings may presently be employed only in electron tube types wherein some heater-cathode leakage current may be tolerated.

The ball milled mixture is dispersed in methanol, sprayed on the heater Wire, and subsequently red in a hydrogen latmosphere. An example of a single blackened insulating coating comprises a mixture of 5% tungsten to 95% aluminum oxide, by weight. The hydrogen firing is for five minutes at 1700 C.

Other methods of application, such as drag coating, cataphoretic coating, and the like are equally adaptable as the methods described for applying the tungsten coatings and mixtures.

What is claimed is:

A heater adapted to be used in a cathode of an electron tube, said cathode comprising a hollow member in which said heater is to be received, said heater comprising a wire structure having an insulating coating thereon comprising a first layer consisting of aluminum oxide in contact with portions of said wire structure and a second layer covering substantially all of said rst layer, said second layer comprising an admixture of aluminum oxide and metallic tungsten particles.

Reerenees Cited by the Examiner UNlTED STATES PATENTS DAVID I. GALVIN, Primary Examiner.

ARTHUR GAUSS, JAMES D. KALLAM, GEORGE N.

WESTBY, Examiners. 

